1. Field of the Invention
The present invention relates to an organic electroluminescent (“EL”) device, and more particularly, to an organic EL device having an improved structure for forming an organic material layer in a unit pixel to a uniform thickness by inkjet printing, and a method of manufacturing the organic EL device.
2. Description of the Related Art
The display market is rapidly growing owing to development and application of new products. Among various displays, a liquid crystal display (“LCD”) is a common flat display because of its light weight and low-power consuming characteristics. It is predicted that the LCD will be in great demand in the market for a considerable time.
However, the LCD is not a self-luminescent device but rather a light-receiving device. Therefore, development of a new flat display that obviates the technical limitations and disadvantages of the LCD such as limited brightness, limited contrast, limited viewing angle, size, etc., is ongoing. An organic electroluminescent (“EL”) device, a new type of flat display, has a good viewing angle and contrast when compared with the LCD since the organic EL device is self-luminescent. Further, since the organic EL device does not require a backlight, the organic EL device consumes less power and can be formed into a light and slim shape. Furthermore, the organic EL device has a fast response time and can be operated using a low voltage. In addition, since the organic EL device can be made from fewer parts spaced closer together, it has a good impact resistance and a wide operating temperature range. Perhaps most importantly, the organic EL can be manufactured at low cost. The organic EL device has a structure formed by an anode, a cathode, and an organic thin layer interposed between the anode and the cathode. The anode is formed of indium tin oxide (“ITO”), and the cathode is formed of a metal having a low work function. The organic thin layer has a thickness of about 100 nm to about 200 nm. Although the organic thin layer can be formed of a single material, generally, the organic thin layer is formed into a multiply layered structure layer using a plurality of organic materials. Further, the organic thin layer is doped with a fluorescent dye or a phosphorous dye to increase its radiating efficiency.
The organic EL device emits light when a hole (the absence of an electron in the valence band of an atom), injected from an anode, and an electron, injected by the cathode, combine to form an exciton. When the exciton de-excites it emits a photon with an energy corresponding to the energy difference between the highest occupied molecular orbital (“HOMO”) and the lowest un-occupied molecular orbital (“LUMO”). When the exciton de-excites an emitting layer (“EML”) it is more likely to emit a photon in the desired visible wavelength range.
Since the mobility of holes is different from that of electrons in an organic material, a hole transport layer (“HTL”) and an electron transfer layer (“ETL”) are used to effectively transfer holes and electrons to an EML. For this reason, the organic EL device is usually formed into a multiple thin layer structure.
When the densities of electrons and holes are balanced in the EML owing to the above-described structure, the radiating efficiency, or the percentage of hole and electron recombinations that result in the emission of a visible wavelength photon, of the organic EL increases. Further, since electrons injected from the cathode toward the EML are confined in the EML by an energy barrier existing at the interface between the HTL and EML, recombination efficiency of electrons and holes increases. The electrons become trapped in the EML where their recombination with a hole will be more likely to result in the emission of a visible wavelength photon. The energy barrier at the EML/HTL interface prevents electrons from traveling to the anode where their recombination with a hole would be less likely to result in emissive radiation.
Furthermore, a recombination region can be moved further away from the cathode than an exciton diffusion length (about 10 nm to about 20 nm) by adjusting the thickness of the ETL to about several tens of nanometers. In this case, extinction of excitons by the cathode can be prevented, thereby increasing radiating efficiency.
In some cases, a hole injection layer is additionally formed between the anode and the HTL using a conductive high-molecular weight material such as copper (II) pthalocyanine (“CuPc”), or other similar substances, so as to lower the energy barrier of hole injection. This means that holes may be injected using less energy. A material such as LiF may be formed between the cathode and the ETL to a thickness of about 0.5 nm to about 1 nm so as to improve electron injection. These layers thereby increase radiating efficiency and decrease a driving voltage of the organic EL device.
FIG. 1A is a schematic perspective view of a conventional organic EL device, and FIG. 1B is a cross-sectional view taken along line A-A′ of FIG. 1A. Referring to FIGS. 1A and 1B, the conventional organic EL device includes a bank 4 defining a plurality of unit pixels 3 on a substrate 2, and an organic luminescent layer 6 formed in the unit pixels 3 by inkjet printing.
In manufacturing the conventional organic EL device using inkjet printing, the bank 4 is formed to confine applied organic material ink in the unit pixels 3. The bank 4 prevents overflowing of the applied organic material ink from one unit pixel 3 to neighboring unit pixels 3. In inkjet printing, ink is applied to fill the unit pixels 3 and contact the bank 4. After that, when the volume of the ink reduces as solvent of the ink evaporates, the ink filled in each of the unit pixels 3 is deformed by surface tension, such that the outer region of the ink becomes thinner. Further, the ink sticks to an inner surface of the bank 4 by an attractive force between the ink and the inner surface of the bank 4. Therefore, as shown in FIG. 1B, the thickness of the organic luminescent layer 6 in the unit pixel 3 decreases from the bank 4 and then increases rapidly toward a central area of the organic luminescent layer 6 then decreases rapidly moving away from a central area of the organic luminescent layer 6 only to increase again as it approaches the bank 4. The resulting structure thus has a thick central area surrounded by a thin ring, which is in turn encompassed by a slightly thicker ring adjacent to the bank 4. The thinner region of the ink filled in the pixel 3 causes problems such as an inter-layer short circuit. Further, an electric field will tend to concentrate on the thinner region once a common electrode is formed on the organic luminescent layer. In such an organic luminescent layer 6 most of the current will pass through the one thin region which will wear out more rapidly than if the current were instead passed throughout the organic luminescent layer 6, thereby decreasing the life span of the entire organic EL device.